If I recall, the first Da Capo had 2 seasons, and both had 26 episodes each.
Da Capo II also had 2 seasons, but they were only 13 episodes each. I think this story takes place 57 years after the first show concludes, and has the main male character from the first Da Capo as an old grandfather by this point, although he's not seen much nor does he play much of a role until the second season.
Da Capo 3 from what I hear is 13 episodes. From what they've said in the show, its about 20 years after Da Capo 2.
You may want to watch them to get some background info on characters in Da Capo 3, but from what I've watched, it's not critical if you don't see them. And I wouldn't blame you if you didn't want to watch it, as that's 78 episodes you'd have to catch up on. There is some continuity between them, namely the cherry blossoms, a girl named Sakura, and a guy named Suginami, who looks and sounds exactly the same in each version (yeah yeah, they used the same voice actor, but it's still unusual to see him/his descendants virtually unchanged in 70+ years lol).
As for where to see them, google is your friend, that's all I'll comment on
Online 
Offline 
